It is known to subject various materials to microwave energy in the cavity of a microwave unit for various purposes, e.g., it being known to employ such technique in connection with the treatment of tobacco in cut fiber or shred form for expanding the same. Presently employed apparatus for this purpose usually includes a conveyor belt to transport the material to and from the cavity, together with a choke at inlet and outlet to absorb any microwaves leaking from these openings, which energy if the openings are large enough for practical operating volumes, will otherwise usually far exceed the safe value of 10mw/cm.sup.2 at a distance of 2 inches established by the Bureau of Radiological Health, U.S. Dept. of Commerce. The choke is made up of energy-absorbing materials that are cooled by flowing water thereto to carry away the heat. Such an arrangement is expensive to install and to operate. Further it does not always give the required protection as shown in Table 1, from Eure, John A., James W. Nicolls, and Robert L. Elder, Am. J. Public Health, 62, 157377, December 1972.
A second disadvantage of such known system is that the microwave energy that passes through the cavity opening and is absorbed in the choke represents an energy loss or deviation from the intended purpose of the microwave cavity operation. This loss can be a significant fraction of the total energy supplied to the cavity. Such a loss can be prevented, however, by the application of the cut-off tube principle, viz., if, in a wave guide with perfectly conducting walls, a possible mode of transmission is excited at a frequency that is below cut-off for that particular mode, no energy will be transmitted through the guide from the point of excitation. This type of waveguide or cut-off tube does not require water cooling because it does not absorb any microwave energy. Such waveguides or cutoff tubes as are effective to the intended purpose generally are of relatively small cross-section so that such a cutoff tube will not allow passage of most shredded or fibrous particulate matter, even in a vertical position unless some form of material conveying assistance is provided. Accordingly, it is desirable that a more effective means of conveying material into the microwave unit be provided, but yet such means as embodies therewith an effective means of prevention of microwave energy escape to the environment outside the microwave unit.